1. Field of the Invention
The present invention relates to a power supply unit which receives an AC power supply and supplies an output of a self-contained power transformer as a power source.
2. Description of the Prior Art
In a power supply unit which receives an AC power supply as an input source or an electronic apparatus which incorporates such a power supply unit, at least a power switch and a power transformer are contained therein and those parts are usually connected through electric wires.
FIG. 1 shows a prior art power supply unit which is designed to meet a North American Safety Standard. A plug receptacle 2, a power switch 3 and a transformer 4 are fixed on a chassis 1 of an electronic apparatus. A plug 6 which is one end of a power supply cord having the other end thereof connected to an AC power supply is plugged in and out to and from the plug receptacle 2. Numeral 7 denotes a filtering capacitor. One wire 8 of at least two wires connected to the plug receptacle 2 is connected to the transformer 4 and the other wire 9 is connected to one terminal of the power switch 3, and a wire 10 connected to the other terminal of the power switch 3 is connected to the transformer 4 so that the AC power is supplied to the transformer 4 and the power switch 3. The transformer 4 is connected to a printed circuit board 11 fixed to the chassis 1 through a wire 12, and the capacitor 7 is also connected to the printed circuit board 11 through a wire 13.
In the illustrated example, the wires 8, 9 and 10 are double insulated wires in order to meet the North American Safety Standard, and those wires must be covered by tubes 14, 15 and 16 in order to assure safety of those portions which might contact a metal part, that is, the transformer 4.
Tubes 17 and 18 are required to fix and prevent detachment of soldered portions of the wires 9 and 10 and the power switch 3, and tubes 19 and 20 are required to prevent the wires soldered to the printed circuit board 11 from contacting the metal part of the power transformer when the wires are detached.
Thus, the prior art power supply unit needs many wire connections and the use of double insulated wires for the purpose of safety and the application of the tubes to prevent the movement of the wires is required because a high AC voltage is applied to those wires. Accordingly, a number of parts and a number of assembling steps increase, and hence manufacturing cost increases.